Deprecate QuickTemplate::msgHtml & fix phan-taint-warning in includes/skins

Deprecate QuickTemplate::msgHtml & fix phan-taint-warning in includes/skins

QuickTemplate::msgHtml() (And the weird override that does the
same thing a different way - BaseTemplate::msgHtml()) are inherently
unsafe as they echo out a raw html message. This is strongly
discouraged in modern code. According to codeSearch tool, nothing
uses these methods, and there is a "@private" annotation on the
QuickTemplate::msgHtml() docblock. Thus hard deprecating it.

Change-Id: I4e9e157e922a36787adef4d0bf7608605c27f0c4